1. Field of the Invention
This invention relates to a method for noncontacting tension measurement in a paper web.
Furthermore, the invention concerns an apparatus for the implementation of the method.
2. Description of Background Art
In paper and printing machinery, the measurement equipment for tension and distribution of tension in the paper web play an extremely decisive role in the reliability of machine operation. Web breaks are difficult to anticipate and cause considerable additional costs, especially since web speeds in paper mills are continually increasing with the web speed reaching 100 km/h. In this respect, a web tension measurement apparatus would be extremely necessary, especially if the distribution of tension could be measured in the cross direction of the machine because practical experience tells that the tension profile varies from very high peaks to fully slack valleys. If tension information is available as an electronic signal, the machine operation can be controlled accordingly, for instance, aiming to avoid web breaks. Today, the preferred methods are based on, for instance, control of rotation speed differences between the rolls, but owing to the slip, this control method of rotation speed is incapable of providing direct tension information from the web.
To date, tension measurement on a web is possible to accomplish by three different methods. The first method is based on blowing a dent on the web by compressed air and then measuring the dent depth. The second method measures the mutual pull force between the rolls by means of force transducers mounted to the shafts of the rolls. The third method utilizes membrane waves in the web, which are detected by microphones installed close to the web. The third method is described in more detail in FI patent publication 62 419. According to the patent, membrane waves are induced in the web by, e.g. a loudspeaker, which propagate in the direction of the applied tension in both the direction the paper is running as well as the counterdirection. The velocity of this membrane wave is then used in determining the web tension by a formula known in physics that expresses tension as proportional to the product of the membrane wave velocity squared and the basis weight of the web. The membrane wave velocity can be measured by microphones mounted at a predetermined distance from the sound source and thereby determining the propagation time of the membrane wave. This method applies vibrations of the acoustic frequency range, such as bursts of 400 Hz frequency. A method based on the use of membrane waves is also described in U.S. Pat. No. 3,854,329. Different from the Finnish method, this publication discloses a method based on the use of ultrasonic vibrations.
The first method compressed air blowing has proved rather inaccurate. In addition, because the method requires the nozzle to be applied very close to the paper web, the hazard of tearing is high.
The second method based on the measurement of mutual pull force between the rolls is relatively commonly used but its disadvantages are, for instance, slow response because the roll masses may be up to hundreds of kilograms. Actually, the method measures the equivalent of tension integrated over the entire width of the web, which leaves, for instance, the tension profile undefined. Furthermore, the tension peaks that break the web remain undetected.
In the third method in accordance with the FI patent, owing to the audio-frequency signal, the microphones, too, must be tuned to the same frequency range, which induces considerable signal-to-noise problems because, as known, the surroundings of a paper machine contain extremely high acoustic noise especially in this frequency range, causing e.g., overload of microphones. In addition, the detection of a burst signal by microphones also detects the direct acoustic signal from the loudspeakers via air, which causes heavy disturbance to the measurement. On the other hand, the practical tests of the method indicate that membrane waves are not capable of propagating in the ultrasonic frequency range, which makes an apparatus described in the US patent impossible to operate.